As well known in the art, there is available an exposure system with the application of voltage (hereinafter called simply voltage-applied exposure) in which, while a photosensitive material having a photoconductive layer on an electrically conductive layer is located in opposition to a charge carrier medium having an insulating layer on an electrically conductive layer, an image is exposed to light with voltage applied between both said electrically conductive layers, thereby forming an electrostatic latent image on the charge carrier medium.
Such voltage-applied exposure is schematically illustrated in FIGS. 1(a)-1(d) wherein reference numeral 1 stands for a charge carrier medium, 1a an insulating layer, 1b a charge carrier medium electrode, 1c an insulating layer support, 2 a photosensitive material, 2a a photoconductor support, 2b an electrode of photosensitive material, 2c a photoconductive layer and E a power source.
The photoconductor support 2a formed of 1-mm thick glass is provided thereon with the transparent photosensitive electrode 2b formed of 1000-.ANG. thick ITO, which is then provided thereon with the photoconductive layer 2c of about 10 .mu.m in thickness to form the photosensitive material 2. With respect to this material 2, there is located the charge carrier medium 1 through an air gap of about 10 .mu.m. The charge carrier medium 1 is formed by the vapor deposition of a 1000-.ANG. thick aluminium electrode 1b on the insulating layer support 1c formed of 1-mm thick glass and the formation of a 10-.mu.m thick insulating layer 10 on this electrode 1b.
As illustrated in FIG. 1a, the charge carrier medium 1 is first located with respect to the photosensitive material 2 through an air gap of the order of 10 .mu.m, and voltage is applied between the electrodes 2b and 1b through the power source E. In a dark place, no change will occur between the electrodes, or uniform discharge will take place between the photoconductive layer 2c and the insulating layer 1a due to a minute dark current flowing through the former during the application of voltage, whereby charges corresponding to the dark current are built up on the insulating layer 1a. This is because the photoconductive layer 2c is of high resistance. Upon incidence of light from the side of the photosensitive material 2, charge carriers (electrons and positive holes) are produced through the region of the photoconductive layer 2c upon which the light strikes, so that major carriers can migrate onto the surface of the photoconductive layer 2c. As a result, discharge takes place between the photoconductive layer 2c and the insulating layer 1a, thus allowing charges to be accumulated on the insulating layer 1a in a quantity corresponding to the exposure dose.
After the completion of exposure, the voltage is turned off, as shown in FIG. 1c, and the charge carrier medium 1 is removed, as depicted in FIG. 1d, to finish the formation of an electrostatic latent image.
When this recording procedure is applied to planar analog recording, high resolution is obtained as is the case with silver salt photography. In addition, although the surface charges formed on the insulating layer 1a are exposed to an air atmosphere, they can be kept without discharge over an extended period of time regardless of whether they are stored in a dark or bright place, because air is a good insulator.
The applicant has already proposed a process for forming an electrostatic latent image using a pre-electrified charge carrier medium or photosensitive material, in which an image is exposed to light while both the electrically conductive layers remain short-circuited.
FIG. 2 illustrates such an image-forming process and FIG. 3 shows the relation between the exposure dose and the potential. In the drawings, reference numeral 3 stands for a charging unit, E a power source and 5 a switch.
A charge carrier medium 1 is first subjected to corona discharge as by applying voltage to a corona wire of the charging unit 3, whereby an insulating layer 1a is charged to a given potential. It is understood that this charging may be achieved either by the application of voltage through a plate electrode or by other means such as frictional or release charging. In this case, the insulating layer may be electrified with charges that are opposite in polarity to the major carriers of the photosensitive material (which are readily transportable). Often, the major carriers are positively charged in the case of an organic photosensitive material whereas, in the case of an inorganic photosensitive material, they are positively or negatively charged depending upon what the material it is formed of. Therefore, the charge carrier medium should be electrified thereon with negative charges, for instance, when the organic photosensitive material is used. Then, the thus electrified charge carrier medium 1 is located with respect to the photosensitive material 2 through an air gap of the order of 10 .mu.m, followed by turning the switch 5 off to short-circuit the electrodes 1b and 2b. Although positive charges opposite in polarity to the negative charges on the surface of an insulating layer are induced on the electrode 1b, some charges are distributed to the electrode 2b, so that there can be a given potential difference between the charge carrier medium and the photosensitive material. For instance, when an image is exposed to light from the side of the photosensitive material in this state, carriers are produced in the photoconductive layer 2c, so that the positive charges can be attracted toward and transported onto the surface of the charge carrier medium. Then, they are coupled to the negative charges ionized in the air gap for neutralization, so that the positive charges ionized in the air gap can be attracted toward the charge carrier medium and neutralized with the negative charges on the surface of the insulating layer. The quantity of the positive charges neutralized with the negative charges on the surface of the insulating layer corresponds to the exposure dose; that is, the potential shown in FIG. 3 is the surface potential of the insulating layer corresponding to the exposure dose. Thus, an electrostatic latent image being formed is tantamount to the surface potential of the insulating layer corresponding to an image. In this case, there is a drop of potential where increased exposure takes place. For instance, the resulting image becomes whitish upon toner development. Thus, the image obtained by this image-forming process is a positive image.
It is understood that when an electrostatic latent image is formed by image exposure according to the process shown in FIG. 2, using thermoplastic resin for the insulating layer 1a, charges opposite in polarity to the surface charges of the resin layer are induced on the electrode 1b. When the charge carrier medium is heated with a heater 7 in this state, as shown in FIG. 4a, the resin layer 1a is so plasticized that the surface of the resin layer is undulated, as shown at 8, by the Coulomb's force between the surface charges of the resin layer and the charges induced on the electrode. Cooling of this causes the undulation to be fixed, as shown in FIG. 4b, giving a positive frosted image.
It is understood that the frosted image may also be formed by forming an electrostatic latent image by usual exposure with the application of voltage and heat-treating it. In this case, however, the frosted image is a negative image.
Because of being characterized by keeping an electrostatic latent image over an extended period of time and rendering analog recording of very high resolution possible, the charge carrier medium is now considered to have various applications. So far, it has been used to record still images, but its application to recording moving images has not come in mind.
Another image exposure process is practiced as well by locating a photosensitive material 10 including a transparent electrode 12 and a photoconductive layer 13 on a support 11 in opposition to a charge carrier medium 20 including an electrode 22 and an insulating layer 23 on a support 21 and applying voltage of a given polarity between the electrodes 12 and 22 through a power source 30. A portion of the photoconductive layer exposed to light is made electrically conductive, and through that portion discharge takes place between the photosensitive material 10 and the charge carrier medium 20, so that charges, e.g., (+) charges can be accumulated on the insulating layer 23 depending upon the exposure dose. At this time, carriers are produced from the portion of the photoconductive layer 13 exposed to light, so that (-) and (+) charges can migrate to the transparent electrode 12 and the surface of the photoconductive layer, respectively. Corresponding to these charges, (-) charges ionized in an air gap are thus accumulated on the surface of the photoconductive layer.
In this way, a still image is formed on the charge carrier medium by voltage-applied exposure but, at the same time, charges of a polarity corresponding to the conditions for forming an image are accumulated on the surface of the photosensitive material as well. For instance, when selenium is used as the photosensitive material, there is such a dark decay characteristic as shown by a characteristic curve A in FIG. 6. When an organic photosensitive material is used, on the other hand, a time in a matter of several tens seconds is needed for decay, as can be seen from FIG. 7a. Hence, when it is intended to pick up images continuously with such a still image-recording process as shown in FIG. 5, this photosensitive material is affected by residual charges, posing a problem that electrostatic images of high quality cannot be recorded.
This invention has been accomplished to provide a solution to the above problems.
An object of this invention is to provide a moving image pickup device which can pick up moving images using a charge carrier medium.
Another object of this invention is to provide a recording medium suitable for picking up moving images.
A further object of this invention is to provide a process for picking up images continuously without causing a residual image to have some adverse influence.